/ Dictionary / Index R radioactinium: Meaning and Definition of Find definitions for: ra•di•o•ac•tin•i•um Pronunciation: (rā"dē-ō-ak-tin'ē-um), [key] — n. Chem. the radioactive isotope of thorium having a mass number 227 and a half-life of 18.8 days. Symbol: RdAc, Th 227 Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. radioacoustics radioactivate Related Content Daily Word Quiz: portent Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language